The present invention relates to an ultrasonic bonding method and an ultrasonic bonding apparatus for connecting bonding wires to metal pads on a semiconductor device.
In a semiconductor device having the form of an ultra-large scale integrated circuit, it is required to connect bonding wires to a large member of metal pads provided at given positions on a substrate of the semiconductor device through bonding to form circuits.
An ultrasonic bonding method using an ultrasonic bonding apparatus is generally utilized for the bonding. In the ultrasonic bonding method, when a bonding wire is bonded to a metal pad on a substrate of a semiconductor device, ball-bonding or wedge-bonding is performed while an ultrasonic vibration having a frequency of 60 kHz or 120 kHz is being applied to the bonding wire.
The most general bonding method in the prior art is a method in which a bonding wire is ball-bonded to a metal pad on a substrate of a semiconductor device while an ultrasonic vibration having a frequency of 60 kHz is being applied to the bonding wire. In an ultrasonic bonding apparatus which is used for such ultrasonic bonding, the maximum vibration amplitude of the bonding tool tip end of the apparatus is above 5 .mu.m.
In the case of using such an ultrasonic bonding apparatus, it is preferable to use a gold (Au) wire having a diameter of 25 .mu.m to 38 .mu.m as the bonding wire; and the ultrasonic bonding is performed under a condition in which the connecting portion (work) is pre-heated above 200.degree. C., and the vibration amplitude of the bonding tool tip end is selected to be 2 .mu.m to 3 .mu.m in order to perform high strength ultrasonic bonding.
Another ultrasonic bonding method in the prior art is a method in which a bonding wire is wedge-bonded to a metal pad on a substrate of a semiconductor device while an ultrasonic vibration having a frequency of 120 kHz is being applied to the bonding wire. In an ultrasonic bonding apparatus used in such ultrasonic bonding, the maximum vibration amplitude of the bonding tool tip end of the apparatus is above 4 .mu.m.
In the case of using the ultrasonic bonding apparatus, the vibration amplitude of the bonding tool tip end is selected to be 2 .mu.m to 3 .mu.m in order to perform high strength ultrasonic bonding, the same as the vibration amplitude employed in the general bonding method.
The principle of the ultrasonic bonding in the ultrasonic bonding method of the prior art is as follows. The bonding is performed by effecting contact between two materials in solid state which are to be bonded to each other, that is, by causing a metallic material forming the bonding wire, for example, a gold (Au) wire, to come into contact with a metallic material forming the metal pad, for example, an aluminum (Al) pad, and then applying frictional vibration to the gold (Au) wire to bond it to the aluminum (Al) pad.
As the materials are pressed into contact with each other, a metallic coupling is produced between the metallic materials so that these metallic materials become attached to each other closely up to an atomic interval level, while a bonding interruption substance and an oxide film attached on the bonding surface portions of the two metallic materials are being removed, whereby the desired bonding is accomplished.
In general, the metal pad provided on the semiconductor device, such as an ultra-large scale integrated device, that is, the aluminum (Al) pad, is an aluminum (Al) alloy film having a thickness of several thousands .ANG. to several .mu.m provided on a silicon dioxide (SiO.sub.2) insulator film, which has a thickness of several hundreds A to one thousand and several hundreds .ANG. and is provided on a silicon (Si) semiconductor substrate.
In the aluminum (Al) pad having such a construction, there are some cases where silicon (Si) is deposited inside the aluminum (Al) alloy film depending on the kind of aluminum (Al) alloys, and nodules of silicon (Si) are formed in the aluminum (Al) pad by the deposition of silicon (Si).
In a case where a gold (Au) bonding wire is ball-bonded to an aluminum (Al) pad through ultrasonic bonding, the bonding is performed by applying a vibration to the gold (Au) wire and the aluminum (Al) pad while a ball portion formed at the end portion of the gold (Au) wire is pressed to the surface of the aluminum (Al) pad using the ultrasonic bonding apparatus.
While performing the ultrasonic bonding, a high shear stress and an internal strain are applied to the internal portion of the aluminum (Al) alloy film, which is being pressed by the ball portion, and the interface portion of the aluminum (Al) alloy film and the silicon dioxide (SiO.sub.2) insulation film.
When the magnitudes of the shear stress and the internal strain exceed certain values, micro-cracks are produced inside the silicon dioxide (SiO.sub.2) insulation film, and in an extreme case cracks are produced inside the silicon (Si) semiconductor substrate.
In a case where cracks are produced inside the silicon dioxide (SiO.sub.2) insulation film or inside the silicon (Si) semiconductor substrate, if nodules of silicon (Si) are formed in the aluminum (Al) alloy film, shear stress, which is produced in the aluminum (Al) pad when the aluminum (Al) pad is pressed by the gold (Au) ball, is concentrated at the peripheral portion of the nodule of silicon (Si), and fine cracks are easily produced in the silicon dioxide (SiO.sub.2) insulator film, even if the pressing force for a friction vibration is comparatively small and is under a low loading condition.
Further, in a case where the silicon dioxide (SiO.sub.2) insulator film is thinner than a given thickness, or in a case where the silicon dioxide (SiO.sub.2) insulator film is of a multi-layer structure having a layer of a different kind of material formed under it, fine cracks are easily produced in the silicon dioxide (SiO.sub.2) insulator film, even if the pressing force for the friction vibration is comparatively small and is under a low loading condition, as described above.
In a case where fine cracks are formed in the silicon dioxide (SiO.sub.2) insulator film, the fine cracks are apt to produce large cracks so as to cause pealing of the ball bonding after that.
Furthermore, in a case where the semiconductor substrate is made of a comparatively brittle material, such as gallium-arsenic (GaAs), phosphorous-indium (InP) or the like, even if the pressing force for the friction vibration is small at the time of performing the ultrasonic bonding, there is a possibility to produce cracks which are sufficiently large to reach the semiconductor device substrate soon after completion of the ultrasonic bonding.
As described above, in the ultrasonic bonding method of the prior art, there are problems in that cracks are produced in the silicon dioxide (SiO.sub.2) insulator film provided on a substrate of a semiconductor device, so that the various kinds of characteristics necessary for the semiconductor device are damaged, and then the reliability the semiconductor device is lost or the productivity is decreased.